42 • Marine Minerals: Exploring Our New Ocean Frontier 



Figure 2-1.— Idealized Physiography of a 

 Continental Margin and Some Common Margin Types 



Sea level 



4000 m 



Continental AbysSaJ 



i^ntal margin ri^^;v„.^,;^(3[^4(^i\ 



Atlantic: (trailing edge) 



Nortti Pacific Margin: (collision) 



Souttiern California Borderland: (extentional) 



Soutfi Pacific: (volcanic arcs, trenches, and carbonate reefs) 

 SOURCES: Office of Tectinology Assessment, 1987; R.W. Rowland, M.R. Goud, 

 and B.A. McGregor, "Ttie U.S. Exclusive Economic Zone— A Summary 

 of its Geology, Exploration, and Resource Potential," U.S. Geologi- 

 cal Survey Circular 912, 1983. 



and the west coast (figure 2-1). These regions have 

 relatively narrow continental shelves and their on- 

 shore geology is dominated by igneous intrusives 

 and volcanic rocks. These rocks supply the thin ve- 

 neer of sediment overlying the continental crust of 

 the shelf Further offshore, the Pacific coast EEZ 

 extends beyond the shelf, slope, and rise to the 

 depths underlain by oceanic crust. In the Pacific 

 northwest, these depths encompass a region of 

 seafloor spreading where new oceanic crust is form- 

 ing. This region includes the Gorda Ridge and pos- 

 sibly part of the Juan de Fuca Ridge, which are 

 located within the U.S. EEZ off California, Ore- 

 gon, and Washington. 



The trailing edge of a continent has a passive 

 margin because it lacks significant volcanic and seis- 

 mic activity. Passive margins are located within 

 crustal plates at the transition between oceanic and 

 continental crust. These margins formed at diver- 

 gent plate boundaries in the past. Over millions of 

 years, subsidence in these margin areas has allowed 

 thick deposits of sediment to accumulate. The At- 

 lantic coast of the United States is an example of 

 a trailing edge passive margin. This type of coast 

 is typified by broad continental shelves that extend 

 into deep water without a bordering trench. Coastal 

 plains are wide and low-lying with major drainage 

 systems. The greatest potential for the formation 

 of recoverable ore deposits on passive margins re- 

 sults from sedimentary processes rather than recent 

 magmatic or hydrothermal activity. 



The Gulf of Mexico represents another type of 

 coast that develops along the shores of a continen- 

 tal sea. These passive margins also typically have 

 a wide continental shelf and thick sedimentary de- 

 posits. Deltas commonly develop off major rivers 

 because the sea is relatively shallow, is smaller than 

 the major oceans, and has lower wave energy than 

 the open oceans. 



Plate edges are not the only regions of volcanic 

 activity. Mid-plate volcanoes form in regions over- 

 lying "hot spots" or areas of high thermal activ- 

 ity. As plates move relative to mantle "hot spots," 

 chains of volcanic islands and seamounts are formed 



